In an optical transmission technique using a wavelength division multiplexing (WDM), a plurality of optical signals of wavelength channels are multiplexed and then transmitted through a single optical fiber. Accordingly, it is anticipated that the WDM technique will be prevailed as one of major optical communication techniques in the near future because it is more cost-effective to apply additional wavelengths than install additional cables and it is suitable for high-capacity high-speed network.
Generally, the optical transmission system supporting the WDM efficiently uses limited optical wavelength channels for multiplexing and demultiplexing optical signals by means of a predetermined wavelength division multiplexing element provided in the optical transmission system. Recently, the multiplexing techniques, for example a coarse wavelength division multiplexing (CWDM) and a dense wavelength division multiplexing (DWDM), which have a more compact channel spacing, are now used as an optical transmission system according to the trend requiring high-capacity high-speed data transmission.
In the early WDM systems, only two wavelength ranges, for example 1,310 nm and 1,550 nm were used for the optical transmission, and a standard single mode optical fiber (i.e. a standard single mode fiber (SSMF)) was used as a transmission media. But since such early systems merely allot an upstream and downstream data to 1,310 nm and 1,550 nm, they were unsuitable for the high-capacity high-speed data transmission in the current level of 10 Gb/s, and limitedly used only for a passive optical network (PON) or the like.
In the case of the CWDM described later, the system was arranged to use a channel spacing of 20 nm as stated in ITU-T G.694.2, and to use an O-band (1,260 to 1,360 nm wavelength range) and a C-band (1,530 to 1,565 nm wavelength range) upon expansion of a local loop. In addition, the accessible wavelength ranges, for example an S-band (1,460 to 1,530 nm wavelength range), an L-band (1,565 to 1,625 nm wavelength range) and so on may be used in the CWDM, but an E-band (1,360 to 1,460 nm wavelength range) may substantially not suitable for transmission with the conventional SSMF due to at most 2 dB/km of high absorption loss by hydroxyl group (OH) existing in a wavelength near to 1,383 nm.
DWDM and CWDM are significantly different to each other in that DWDM has narrower channel spacing than CWDM. Generally, the channel spacing is normalized as an standard according to ITU-T G.694.1, for example 0.4 nm, 0.8 nm, 1.6 nm, 3.2 nm, etc. In general, the wavelength ranges of the C-band and the L-band were widely used in such DWDM, but the optical wavelength of the E-band may not used for transmission with the conventional SSMF due to high absorption loss peak by hydroxyl group (OH) as in the CWDM.
Meanwhile, in a paper “Fabrication of Completely OH-Free VAD Fiber” (Electronic Letters, 16(19), 1980), it is shown that it is important for SMF to satisfy G.652.C in a WDM system, but there were not described dispersion characteristics of the optical fiber for optical transmission of the E-band, as well as an information about channel allotment within loss spectrum for the optical fiber.
U.S. Pat. No. 5,680,490 discloses a splitting system which divides channels to transfer optical signals at multiple wavelengths using WDM by two steps. As an example, it was proposed that if 4 optical signals were transferred in the CWDM using an optical fiber, they were received in DWDM, allowing the 4 channels to be divided into a number of channels.
Also, U.S. Pat. No. 6,205,268 discloses a technique concerning a high-capacity optical fiber network that operates at the E-band. According to the patent, there is proposed an optical transmission system for inhibiting stimulated Raman scattering (SRS) interference between WDMs or analog signals and non-linear phenomena such as 4 phonon mixing (4 PM) between WDM signals, and also supporting WDM operation of 1.4 mm wavelength range (1,335 to 1,435 nm).
However, the transmission media, which may optimally maintain optical loss properties and dispersion characteristics at E-band, should be actually provided to use the channel wavelength signal of the E-band in the multiplexing technique. Also an elaborate and inexpensive transmission system, which may carry out a stable multiplexing operation even in CWDM or DWDM having a relatively narrower channel spacing than WDM, has been required. However, it was difficult to use the wavelength channels of the E-band together with those of other bands since both wavelength channels of the E-band and other bands might not be multiplexed together due to absence of such transmission system in the prior art. Therefore, the conventional transmission system has a problem that it is difficult to increase the data-transmitting capacity or construct a compatible optical network in a suitable manner.